Recently, improving fuel economy of automobiles has been an important issue from the viewpoint of global environmental conservation. Accordingly, there have been active movements to reduce the weight of car bodies by increasing the strength of the material of car bodies to thereby allow the thickness of the material to be reduced. However, an increase in the strength of a steel sheet results in deterioration of formability. Thus, currently, development of a material having both high strength and high formability has been anticipated. Moreover, when the high strength steel sheet is formed into a complicated shape such as that of automobile components, the occurrences of cracking and necking at an overhanging portion and a stretch flange portion are big issues. Thus, a high strength steel sheet having both high ductility and good hole expandability, the steel sheet being capable of overcoming the issues of the occurrences of cracking and necking, has been needed. Furthermore, increasing the strength of a steel sheet and reducing the thickness of a steel sheet significantly deteriorate shape fixability. To address this, in press-forming, a method of designing a die while expecting there to be a change in shape after releasing, which is estimated in advance, has been widely employed. However, when the tensile strength (TS) of a steel sheet varies, the deviation from the change in shape estimated by assuming a constant TS increases. Consequently, defects in shape occur and, therefore, individual reworking operations after the press-forming such as shaping by sheet metal processing, are necessary. As a result, mass production efficiency is significantly reduced. Thus, the TS variation of a steel sheet is required to be as small as possible.
For example, Japanese Unexamined Patent Application Publication No. 61-157625 proposes a high strength steel sheet having a tensile strength of 1000 MPa or more, a total elongation (EL) of 30% or more, and markedly high ductility, the steel sheet being manufactured using strain induced transformation of retained austenite. Such a steel sheet is manufactured by causing a steel sheet containing C, Si, and Mn as fundamental components to form austenite, subsequently quenching the steel sheet in the bainite transformation temperature range, and performing isothermal holding, namely, an austempering treatment. Retained austenite is formed due to the concentration of C in austenite caused by the austempering treatment, and formation of a large amount of retained austenite requires a large amount of added C exceeding 0.3%. However, an increase in the C concentration in steel deteriorates spot weldability and, in particular, a C concentration exceeding 0.3% significantly deteriorates spot weldability. Thus, it has been difficult to bring this technique into active use for steel sheets for use in automobiles. In addition, no consideration is given to hole expandability and stability of mechanical properties in JP '625 because the main purpose is to improve the ductility of a high strength thin steel sheet.
In Japanese Unexamined Patent Application Publication No. 1-259120, a good strength-ductility balance is achieved by heat-treating a high Mn steel in the ferrite-austenite dual-phase region. However, in JP '120, no analysis is performed on the improvement of ductility caused by concentrating Mn in untransformed austenite. Thus, there is a room to improve formability. In Japanese Unexamined Patent Application Publication No. 2003-138345, local ductility is improved by hot-rolling a high Mn steel, thereby causing a microstructure to include bainite and martensite, performing annealing and tempering to form fine retained austenite, and causing the microstructure to further include tempered bainite or tempered martensite. However, it is difficult to maintain strength because of the microstructure including a large amount of bainite or martensite formed by tempering at a high temperature. In addition, the amount of retained austenite is limited to improve the local ductility, which results in an insufficient amount of total elongation.
It could therefore be helpful to provide a high strength steel sheet having a steel composition including a low C content, a TS of 780 MPa or more, a TS×EL of 22000 MPa·% or more in addition to excellent hole expandability and stability of mechanical properties and a method of manufacturing the steel sheet.